Re: [PATCH v3 28/46] kmsan: entry: handle register passing from uninstrumented code

[Alexander Potapenko]

On Fri, May 6, 2022 at 6:14 PM Thomas Gleixner <tglx@xxxxxxxxxxxxx> wrote:
>
> On Fri, May 06 2022 at 16:52, Alexander Potapenko wrote:
> > On Thu, May 5, 2022 at 11:56 PM Thomas Gleixner <tglx@xxxxxxxxxxxxx> wrote:
> >> @@ -452,6 +455,7 @@ irqentry_state_t noinstr irqentry_nmi_en
> >>         rcu_nmi_enter();
> >>
> >>         instrumentation_begin();
> >> +       unpoison(regs);
> >>         trace_hardirqs_off_finish();
> >>         ftrace_nmi_enter();
> >>         instrumentation_end();
> >>
> >> As I said: 4 places :)
> >
> > These four instances still do not look sufficient.
> > Right now I am seeing e.g. reports with the following stack trace:
> >
> > BUG: KMSAN: uninit-value in irqtime_account_process_tick+0x255/0x580
> > kernel/sched/cputime.c:382
> >  irqtime_account_process_tick+0x255/0x580 kernel/sched/cputime.c:382
> >  account_process_tick+0x98/0x450 kernel/sched/cputime.c:476
> >  update_process_times+0xe4/0x3e0 kernel/time/timer.c:1786
> >  tick_sched_handle kernel/time/tick-sched.c:243
> >  tick_sched_timer+0x83e/0x9e0 kernel/time/tick-sched.c:1473
> >  __run_hrtimer+0x518/0xe50 kernel/time/hrtimer.c:1685
> >  __hrtimer_run_queues kernel/time/hrtimer.c:1749
> >  hrtimer_interrupt+0x838/0x15a0 kernel/time/hrtimer.c:1811
> >  local_apic_timer_interrupt arch/x86/kernel/apic/apic.c:1086
> >  __sysvec_apic_timer_interrupt+0x1ae/0x680 arch/x86/kernel/apic/apic.c:1103
> >  sysvec_apic_timer_interrupt+0x95/0xc0 arch/x86/kernel/apic/apic.c:1097
> > ...
> > (uninit creation stack trace is irrelevant here, because it is some
> > random value from the stack)
> >
> > sysvec_apic_timer_interrupt() receives struct pt_regs from
> > uninstrumented code, so regs can be partially uninitialized.
> > They are not passed down the call stack directly, but are instead
> > saved by set_irq_regs() in sysvec_apic_timer_interrupt() and loaded by
> > get_irq_regs() in tick_sched_timer().
>
> sysvec_apic_timer_interrupt() invokes irqentry_enter() _before_
> set_irq_regs() and irqentry_enter() unpoisons @reg.
>
> Confused...

As far as I can tell in this case sysvect_apic_timer_interrupt() is
called by the following code in arch/x86/kernel/idt.c:

  INTG(LOCAL_TIMER_VECTOR,                asm_sysvec_apic_timer_interrupt),

, which does not use IDTENTRY_SYSVEC framework and thus does not call
irqentry_enter().

I guess handling those will require wrapping every interrupt gate into
a function that performs register unpoisoning?

By the way, if it helps, I think we don't necessarily have to call
kmsan_unpoison_memory() from within the
instrumentation_begin()/instrumentation_end() region?
We could move the call to the beginning of irqentry_enter(), removing
unnecessary duplication.